CT systems produce planar images along imaginary cuts through a subject. Each cut is referred to as a slice. Scanners comprise an X-ray source which revolves about an imaginary axis through a subject. The X-rays after passing through the subject impinge on an opposing array of detectors, which may also be revolving. Data for reconstruction of a single image includes a set of views corresponding to different projection angles, each view comprising radiation intensity data measured by detector elements.
A prior art CT study of a subject for obtaining successive planar images includes the following steps:
1. Putting the patient on the bed in a CT system gantry. PA1 2. Scanning the patient. The scan includes the revolution of the X-ray source about the subject and acquisition of radiation intensity data per detector element per angle of revolution of the X-ray source. PA1 3. Reconstructing the image. Optional display, archiving and/or filing. PA1 4. Incrementing the bed to the position of the next slice. PA1 1. P. Slavin, U.S. Pat. No. 3,432,657 (1969). PA1 2. I. Mori, U.S. Pat. No. 4,630,202 (1986). PA1 3. H. Nishimura, U.S. Pat. No. 4,789,929 (1988). PA1 4. W. A. Kalander, P. Vock and W. Seissler in Advances in CT (Springer-Verlag, Berlin, Heidelberg 1990, pp. 55-64). PA1 5. C. R. Crawford and H. F. King, Med. Phys. 17(6), (1990) pp. 967-982 and references therein. PA1 1. Linear and non-linear interpolation schemes; PA1 2. Variable bed speeds associated with appropriate interpolation schemes; and PA1 3. Reducing the bed speed so that the bed moves less than a whole slice width within a single gantry revolution. PA1 a gantry, PA1 a bed for supporting a scanned subject, PA1 an X-ray source mounted on said gantry, PA1 means for rotating the X-ray source about the subject, PA1 an X-ray detector array on a side of the subject opposite to the X-ray source, PA1 said detector array comprising means for simultaneously detecting X-rays that have traversed multiple planar sections of said subject to acquire radiation density data, PA1 means for causing relative motion in an axial direction between the bed and the gantry while the X-ray source is revolving about the subject, and PA1 means for reconstructing images from said data where said reconstructing includes means for reformatting the acquired data into single plane data interpolating between data measured at different planar sections through said subject.
Steps 2-4 are repeated as long as more slices are required. Step 3 may be concurrent with steps 2 and 4, but step 4 must be successive to step 2. Step 4 involves acceleration and de-acceleration of the bed as the bed must be stationary during the scan when successive planar images are acquired. Step 2 may involve acceleration and de-acceleration of the gantry to the proper rotational speed. Gantry acceleration and deceleration may, however, be circumvented by using a continuous rotation scanner such as provided, e.g., by slip-ring technology.
An ubiquitous problem encountered by CT systems is that heat builds up in the X-ray source as more scans are being performed. In prior art systems, when the scanning rate is such that heat build-up rate is higher than the cooling rate of the X-ray source and the X-ray source is at the limit of allowed stored heat, further scans must be delayed. The invention of the above listed Patent Application improves the utilization of the X-ray source; since that Application enables a CT scanner to simultaneously scan mutli-planar slices of the subject during single X-ray exposure and, therefore, unmanageable heat build-up is less likely to occur.
Other problems occur with the prior art CT scanners used to obtain a series of planar images. For example, the successive nature of the scanning process described hereinabove, prolongs the time during which the subject is imaged. The longer throughput time results in greater patient discomfort. The bed acceleration and de-acceleration add to the discomfort of the patient. Further, the patient is required to adjust his breathing cycle to the scanning rate so as to reduce motion related image artifacts. When the the examination period is longer, the breath control is more difficult resulting in more patient motion, both during scans and between scans. Patient motion, voluntary and involuntary, between scans decreases the repeatability that is desired between adjacent slices. In particular, oblique reformatting and 3-D images formed from series of planar images are adversely affected.
To overcome these problems, helical or spiral scanning systems are being investigated and developed. This type of scanning is described in the following references:
Essentially, with helical scanning scanners, the subject is continuously scanned while the gantry makes multiple rotations about the subject and the bed is moved relative to the gantry along the axis of rotation simultaneously with the rotation. Images of successive slices are reconstructed from sets of views using well known reconstruction algorithms.
In conventional non-helical; i.e., stationary bed CT scans made to image successive slices, the different views making up the different sets correspond to projections within the same plane. On the other hand, in the helical scans the different views making up the different sets correspond to projections in different planes. Therefore, non-modified conventional reconstruction yields artifacts; i.e., highly distorted images. To prevent such artifacts, the raw data is reformatted before backprojection into single plane data sets by interpolating between data measured at the same gantry angle but at different subject positions, providing data of different planes.
Hereinbelow, the theoretical slice sensitivity profile is defined as the response of the scanner to a small homogeneous object as a function of the object position along the axial direction. The slice width is defined as a full width at half maximum (FWHM) of the slice sensitivity profile.
In stationary-bed CT scans, the slice width is determined by collimators limiting the beam width or the length of the detector elements in the axial direction. In helical scans, data from different planes through the subject are mixed and the slice sensitivity profile is smeared. Therefore, the FWHM of the profile tends to be larger in a helical scan than in a stationary-bed scan for a given collimator setting. Also, the ratio between the full width at tenth maximum (FWTM) and the FWHM of the sensitivity profile, which is a measure of the quality of the slice width, is severly degraded.
Various schemes to improve the slice sensitivity profile in helical scans are discussed in the references cited hereinabove. These Include:
None of these schemes, however, provides images of the quality obtained in prior art stationary bed CT systems for a given radiation dose applied to the subject. Furthermore, because of the increased time length of exposure required in helical scans, the available X-ray intensity is likely to be less than in stationary bed CT systems, thus further decreasing image quality.
In addition to the image quality problems exclusive to prior art helical scan CT systems, the prior art CT systems also encounter motion artifacts and partial volume artifacts. To reduce motion artifacts it is advantageous in CT, In general, to have the scan time as short as possible. However, sometimes it is required to prolong the scan in order to reduce the statistical noise. There exists a technique whereby the gantry revolves multiple revolutions about the scanned subject and the data from the multiple revolutions are averaged before backprojection to yield an image of high statistics and reduced motion artifacts. The technique is disclosed by R. Hupke, in "Advances in CT" (Springer-Verlag, Berlin Heidelberg 1990, pp. 3-15).
Partial volume artifacts result from high spatial frequency variation of the radiation absorption coefficients in the subject. Such artifacts are reduced, in general, by using a smaller slice width. However, sometimes it is advantageous to use large slice widths so a large scanned volume may be covered by fewer slices. There exists a technique whereby data of several consecutive thin slices are averaged before backprojection to form one image. The technique is disclosed in the R. Hupke reference noted hereinabove.
The technique provides images with reduced partial volume artifacts and saves some of the reconstruction time, but it requires several scans to form one image and it is, therefore, inefficient.